1. Field of the Invention
This invention relates to an improved optical pickup device employed for recording/reproducing information on an optical disk.
2. Background Art
FIG. 4A is a diagrammatic perspective view illustrating an optical system separated from a conventional optical pickup device, and FIG. 4B is an enlarged partial side elevation of the optical system shown in FIG. 4A. Referring to these figures, a laser light beam 2 emitted from an optical source 1 is split by a diffraction grating 3 into a zeroth order diffracted light beam 5 for reading signal pits on the disk 4 and for detecting focusing errors and a pair of first order diffracted light beams 5b and 5c for detecting tracking errors.
The three split laser beams 5a, 5b and 5c are passed through a holographic grating 6 and collimated by a collimator 7 into parallel beams which are focused by an object lens 8 to form three corresponding light spots 9a, 9b and 9c on an information surface of a disc 4. At this time, the first order light spots 9b and 9c need be aligned with the tangential direction of the disc 4, that is, with the arraying direction of the signal pits, with the zeroth order light spot 9a as the center of symmetry..
The laser light beams reflected on the information surface of the disc 4 are returned on the same light path and, after passing through the collimator 7, again fall on the holographic grating 6 as they are collimated. The laser light having passed through the holographic grating 6 is separated into laser light beams 11b and 11c for detecting tracking errors and a laser light beam 11a for detecting focusing errors and radio frequency (RF) signals before they are deviated and fall on a photosensor 10.
In the photosensor 10, focusing error signals obtained by a kind of the wedge prism method, RF signals associated with the pits on the disk 4 and tracking error signals obtained by a three beam method, are detected.
In the conventional optical pickup device, when adjusting the direction of alignment of the light spots 9b and 9c on the disk 4 corresponding to a pair of first order light beams 5b and 5c for detecting the tracking errors, it is necessary to rotate the first order light spots 9b and 9c about the zeroth order light spot 9a as center, by rotating the diffraction grating 3 along the direction of arrow marks A and B about the optical axis as center.
However, in the conventional optical pickup device, it is extremely difficult to provide a mechanism for rotating the diffraction grating 3, because of the extremely narrow distance between the light source 1 and the holographic grating 6.
Also the provision of such rotating mechanism results in an increased number of component parts and an increased size and manufacturing cost of the optical pickup device.